The administrative district of Ostrołęka includes, within its boundaries, considerable part of Kurpie Plain which is a geographical subregion of Mazovian Lowland. The Plain, being a wide valley, is preceded by frontal Masurian moraines and covered by sedimentary sands and certain amount of frontal moraine's hills. Monotony of the landscape is diversified by sand dunes - sand dunes located in the east of the Szkwa River, in the neighbourhood of Serafin and Kraska villages and on the left bank of the Omulew River are particularly interesting. The region is being crossed by numerous rivers, including the main one in this part of country, the Narew River. It belongs to small numbers of rivers which remained their natural character alongside its down-stream. Despite of a few towns located on its banks it is one of the rivers with the highest degree of purity. It constitutes an important communication route both, as navigable river in the past and as water route from Warsaw to Masurian lakes nowadays. Among tributaries of the Narew (within the boundaries of the district) the following should be mentioned: the Omulew, Płodowica, Rozoga and Szkwa Rivers. Primitive character of the Omulew River make it an excellent water route for canoeing. Serafin Lake in Łyse Commune and swamps and peabogs in the neighbourhood of Karaska in Kadzidło Commune and Płoszy in Leiis Commune completes the hydrological picture of the region. The area of Kurpie used to be covered by thick primeval forests of which a certain part may be admired nowadays. There are mixed, pine-spruce and pine forests. Additionally there are oaks, hornbeams, birches, aspens. Juniper, which can grow to considerable size, as well as mountain-ash and hazel are often met here. All interested in nature are able to find old-time natural environment in the nature reserves: "Czarnia", "Surowe", "Płoszyce", "Serafin", "Karaska", "Podgorze".